1. Field of the Invention
The present invention relates to a method for fabricating a titanium nitride thin film by CVD, and to a CVD apparatus therefor.
2. Related Art
Reactive sputtering methods featuring the use of metallic titanium targets and nitrogen gas have been primarily used in the past for growing titanium nitride thin films on the substrates of semiconductor devices, various electronic components, various sensors, and the like. In recent years, due to the creation of superfine, large-scale-integration silicon circuits, the design rules for DRAM of 64 Mbit and higher, require dimensions of about 0.35 xcexcm or less, and cause the aspect ratio of contact holes in devices to gradually increase. A drawback of using titanium nitride thin films as barrier metals for such contact holes is that inadequate step coverage results when these titanium nitride thin films are deposited by conventional reactive sputtering. Unsatisfactory step coverage adversely affects the electrical characteristics of contact holes and is expected to pose a serious problem for creating the next generation of devices. It is therefore expected that conformal barrier metals will be formed using CVD techniques, which provide excellent coverage characteristics or filling characteristics.
In view of this situation, attention was attracted in recent years to techniques for fabricating titanium nitride thin films by CVD (chemical vapor deposition). Various CVD techniques and source gases have been proposed for fabricating titanium nitride thin films. One such technique features the use of a tetrakis(dialkylamino)titanium (hereinafter abbreviated as xe2x80x9cTDAATxe2x80x9d), an organometallic compound. The chemical structural formula of TDAAT is shown in FIG. 8. In the chemical structural formula, R is an alkyl group. Tetrakis(diethylamino)titanium (hereinafter abbreviated as xe2x80x9cTDEATxe2x80x9d) is obtained when R is an ethyl group, and tetrakis(dimethylamino)titanium (hereinafter abbreviated as xe2x80x9cTDMATxe2x80x9d) is obtained when R is a methyl group.
These organotitanium compounds, although liquids at room temperature and atmospheric pressure, can be fed into a reaction vessel through a shower head together with carrier gases such as H2, argon, and N2 when vaporized. A mixed gas (such as added gas) chemically reactive with organotitanium compounds is also fed into the reaction vessel. A substrate is disposed within the reaction vessel, and this substrate is kept at a prescribed reaction temperature.
The organotitanium compound and the mixed gas initiate a reaction that yields titanium nitride, which adheres to the substrate as a titanium nitride film. It is known that the step coverage and electrical characteristics of the titanium nitride thus deposited depend on the reaction pressure, the substrate temperature, and the flow rates of the mixed gas and the organotitanium compound reacting inside the reaction vessel.
For example, according to Raajimaker (Thin Solid Films, 247 (1994), 85) and other sources, the source gas TDAAT is fed to a reaction vessel together with argon (carrier gas), and a titanium nitride thin film is fabricated using ammonia gas (NH3) as an added gas. The flow rate of the ammonia gas is 1000 sccm or greater. The resulting titanium nitride thin film delivers adequate step coverage (85%) when deposited in a contact hole having a diameter of 0.8 xcexcm with an aspect ratio of 1. Applied to contact holes with diameters of 0.35 xcexcm, and less, such as those used for 64-Mbit DRAMS, however, this technology is expected to yield a step coverage of merely 20% or less.
Furthermore, according to Jackson et al. (R. L. Jackson, E. J. MCineney, B. Roberts, J. Strupp, A. Velaga, S. Patel and L. Halliday, Proc. Advanced Metallization for ULSI Application, ed. by D. P. Favreau, Y. Shacham-Diamond, and Y. Horiike (Mat. Res. Soc., Pittsburgh, Pa., 1994), p. 20), a source TDEAT is vaporized by being passed through a vaporizer, and the vaporized material is fed together with a carrier gas (nitrogen gas) to a reaction vessel through a shower head. In addition, ammonia gas (NH3) is added through a shower head in a separate conduit, yielding a titanium nitride thin film. In particular, the cited paper reports on the effect demonstrated by the ratio of the feed rates of the source TDEAT and the ammonia gas. The step coverage decreases from 65% to about 20% as the amount in which the ammonia gas is added increases at a film depositing temperature of 350xc2x0 C. and a pressure of 10-50 torr in a contact hole with a diameter of 0.35 xcexcm and an aspect ratio of 3.4. On the other hand, the step coverage for the same contact hole decreases to 5% if the film-depositing temperature is raised to 425xc2x0 C. Thus, the step coverage of fine contact holes becomes inadequate if the flow rate of ammonia gas increases in relation to TDEAT.
Furthermore, Intermann et al. (A. Intermann and H. Koerner, J. Electrochem. Soc., Vol. 140, No. 11 (1993), 3215) published a detailed report on the reasons for the deterioration of step coverage due to the addition of such ammonia gas, concluding that step coverage deteriorates as a result of violent chemical reactions between TDMAT and the ammonia gas.
It is clear that the step coverage of contact holes provided by the conventional techniques described above deteriorates if ammonia gas is added to TDAAT, and this drawback presents a serious challenge to the adoption of techniques in which titanium nitride films are deposited by CVD using organotitanium compounds to the mass production of semiconductor devices. Titanium nitride thin film can also be obtained without the addition of ammonia gas, but this approach is disadvantageous because of the presence of carbon in the films.
An object of this invention is to provide a method and apparatus capable of improving the step coverage of contact holes or grooves when a titanium nitride thin film is fabricated by CVD using TDAAT as a source material.
Another object of this invention is to provide a method and apparatus for suppressing the reactions that yield titanium nitride in the space between the substrate and the gas-supply shower head, and for promoting the reactions that yield titanium nitride on the substrate surface.
According to the present invention, a method for fabricating a titanium nitride thin film in a reaction vessel on a surface of a substrate heated to a prescribed temperature, includes the steps of mixing tetrakis(dialkylamino)titanium and a first carrier gas to create a first mixed gas; feeding the first mixed gas into the reaction vessel through a first set of nozzles, while confining the flow rate of the tetrakis(dialkylamino)titanium to a range of 0.004-0.2 g/min. and confining the flow rate of the first carrier gas mixed with the tetrakis(dialkylamino)titanium to a range of 100-1000 sccm.; mixing an added gas reactive with the tetrakis(dialkylamino)titanium with a second carrier gas to create a second mixed gas; feeding the second mixed gas into the reaction vessel through a second set of nozzles, while confining the flow rate of the added gas to a range of 10-100 sccm. and confining the flow rate of the second carrier gas to a range of 10-500 sccm.; and depositing a titanium nitride thin film by the first mixed gas and the second mixed gas while confining the pressure inside the reaction vessel to a range of 0.1-15 Pa.
A CVD apparatus for fabricating a titanium nitride thin film according to the present invention includes an evacuatable reaction vessel having an interior, a pumping apparatus capable of exhausting the reaction vessel and maintaining the interior of the reaction vessel at a prescribed pressure, a gas feeder for introducing a mixed gas into the reaction vessel, a substrate holder in the reaction vessel for holding a substrate to be coated with a titanium nitride thin film, and a heater for heating the substrate. The gas feeder is equipped with the following components: (a) a vaporizer for vaporizing tetrakis(dialkylamino)titanium from a liquid source material, (b) a first flow controller capable of setting a flow rate of the tetrakis(dialkylamino)titanium to any level within a range of 0.004-0.2 g/min, (c) a second flow controller capable of setting a flow rate of a first carrier gas mixed with the tetrakis(dialkylamino)titanium to any level within a range of 100-1000 sccm, (d) a third flow controller capable of setting a flow rate of an added gas reactable with the tetrakis(dialkylamino)titanium to any level within a range of 10-100 sccm, (e) a fourth flow controller capable of setting a flow rate of a second carrier gas being mixed with the added gas to any level within a range of 10-500 sccm, (f) a first supply conduit for mixing the tetrakis(dialkylamino)titanium and the first carrier gas to create a first mixed gas and guiding the resulting first mixed gas into the reaction vessel, (g) a second supply conduit for mixing the added gas and the second carrier gas to create a second mixed gas and guiding the resulting second mixed gas into the reaction vessel, and (h) a shower head which is provided with a plurality of first nozzles connected to the first supply conduit, and a plurality of second nozzles connected to the second supply conduit, and which is configured such that the first and second mixed gases are fed into the reaction vessel through the nozzles.